The present invention relates to a MOSFET device and a method for manufacturing the same, and more particularly, to a MOSFET device which can suppress a passing gate effect and a method for manufacturing the same.
As the design rule for developing MOSFET (metal oxide semiconductor field effect transistor) devices have been decreasing, a channel length in the recently developed MOSFET devices have also decreased. According to this trend, not only the channel length of a cell transistor serving as a storage unit but also the channel length of the transistor of a peripheral circuit is decreased. Therefore, when attaining the target threshold voltage (Vt) of a MOSFET device required in a specific semiconductor device, limitations necessarily exist in using a conventional planar transistor structure.
In order to overcome this problem, researches have actively been made for a MOSFET device having a three-dimensional recess gate, which is formed in a manner such that grooves are defined by etching a semiconductor substrate and gates are formed in the grooves.
In the MOSFET device having this recess gate structure, since the etched portion of the semiconductor substrate is used as a channel, the channel length of the transistor can be secured, and an effective channel length can be increased.
However, in the MOSFET device having the recess gate, due to the structural characteristics of the recess gate, the electric fields of the adjoining recess gates interact upon each other since the adjoining gates are not electrically screened from each other. As a consequence, the threshold voltage (Vt) decreases by the voltages applied to the adjoining recess gates.
More specifically, a passing gate effect occurs in which a main gate formed in the groove is influenced depending upon the state of a passing gate. The passing gate effect causes the main gate to be influenced by the voltage of the passing gate, as the voltage of the passing gate increases and the potential of the main gate decreases. Consequently, as the influence from the field increases, the threshold voltage (Vt) of the main gate decreases. As a result, the passing gate effect decreases the threshold voltage of the main gate and increases the off-current of the MOSFET device.